The invention concerns a process for preparing thiodiglycolic acid directly from bromoacetic acid, diglycolic acid or glycolic acid by contacting one of these acids or a mixture of these acids and hydrogen sulfide in the presence of aqueous hydrogen bromide.
Thiodiglycolic acid (thiodiacetic acid or thioethanoic acid) has the proposed molecular structure EQU HOOCCH.sub.2 --S--CH.sub.2 COOH
is a white solid and melts at about 130.degree. C. The currently accepted process for preparing thiodiglycolic acid calls for reacting sodium sulfide and sodium chloroacetate followed by acidification. Thiodiglycolic acid can also be prepared together with thioglycolic acid by the reaction of glycolic acid with hydrogen sulfide in the presence of silver oxide. These and other processes in which thiodiglycolic acid is a by-product are discussed in Kirk Othmer, Encyclopedia of Chemical Technology, Vol. 20, pages 198 et seq. (1967) 2nd Edition.
Thiodiglycolic acid salts can be used in hair-waving preparations, particularly the ammonium salt. The calcium salt can be used in depilatory compositions. The color reaction of thioglycolates and diglycolates with iron salts is the basis for qualitative tests for iron.